The disclosure relates to a fuel injection system for an internal combustion engine having an automatic stop-start system, comprising at least one fuel pump, which via at a fuel pump line delivers fuel towards a common fuel rail, to which a fuel return line is connected.
DE 195 47 877 A1 discloses a high-pressure fuel accumulator system for a fuel injection system used in internal combustion engines, in which system fuel is delivered via a high-pressure pump into at least one central main fuel accumulator, from whence it is fed to electrically controlled injection valves. In the event of operating load variations it must be possible to produce considerable pressure changes in the high-pressure accumulator within just a few engine revolutions, DE 195 47 877 A1, for example, explaining that additional fuel could be delivered into the high-pressure fuel accumulator by means of the high-pressure pump. The high-pressure pump would then have to deliver virtually twice the quantity of fuel, and would also have to be designed for this, so that for the greater part of the overall operating time the high-pressure pump would be over-dimensioned. In order to relieve the high-pressure pump, particularly in respect of the load variations, and in order to be able to use a high-pressure pump of smaller design, DE 195 47 877 A1 therefore proposes that an auxiliary fuel accumulator be hydraulically connected to the main fuel accumulator, a remotely operated, electrically controlled valve being arranged in the hydraulic connection between the fuel accumulators. Thus in the event of operating load variations of the internal combustion engine, fuel could be delivered to the main fuel accumulator from the auxiliary fuel accumulator, without the high-pressure pump having to deliver more fuel.
DE 102 23 077 A1 discloses a fuel injection system for internal combustion engines with direct fuel injection. The system comprises a pre-supply pump and a high-pressure pump. The pre-supply pump delivers fuel to the high-pressure pump, which at least indirectly supplies one or more injectors with fuel. A pressure accumulator is provided between the pre-supply pump and the high-pressure pump, the pre-supply pump being cyclically operated as a function of the pressure prevailing in the pressure accumulator. In other words, the pre-supply pump is switched off when a defined pressure prevails in the pressure accumulator.
DE 10 2004 017 729 A1 discloses a fuel injection system for internal combustion engines, comprising a low-pressure area and a high-pressure area. On the low-pressure side the quantity of fuel is controllably matched to the quantity demand by the internal combustion engine by delivering fuel in the low-pressure area from the fuel tank by means of a fuel pump and feeding it via a feed circuit to the suction side of a high-pressure pump. Excess fuel flows back to the fuel tank via a return line. A non-return valve, the function of which will not be described in more detail, is arranged in the return line. A restrictor connection is arranged between the return line and the feed line.
DE 30 44 254 A1 discloses a fuel injection device having a high-pressure accumulator. Pressure relief lines of all injection valves open into a common low-pressure accumulator from which a return line branches off, and in which a fuel pressure level can be set and kept at a specific ratio to that of the high-pressure accumulator. A buffer accumulator chamber is assigned to the fuel path at some point downstream of a pilot valve. The buffer accumulator chamber forms an auxiliary volume, on which a limited elasticity is imposed, and may itself be of pliable design and defined by a hollow body with elastically deformable walls.
Modern internal combustion engines are often designed with so-called automatic stop-start systems so as to shut-off the internal combustion engine in the event of a brief stoppage of the motor vehicle, in order thereby to save fuel when the vehicle is stationary. Suitable mechanisms are used to restart the internal combustion engine when an automatic start is requested. Such automatic stop-start systems afford distinct savings in the fuel consumption of an automobile, for example, and other motor vehicles.
In order to be able to equip common rail diesel engines, for example, with an automatic stop-start system, it is necessary within a short space of time to provide sufficient pressure in the common rail to be able perform the first fuel injection. Since the pressure in the high-pressure system drops due to leakage after stopping the internal combustion engine, the necessary pressure must be built up again during the starting sequence. Starting the engine following a complete or partial stoppage of the engine must then be feasible as rapidly as possible in order to avoid unwanted delays, for example in critical road situations, or to ensure a comfortable driving performance.
For starting a diesel engine having a common rail system, for example, it is necessary to provide a minimum pressure in the rail system. This minimum pressure amounts to 100-150 bar, for example, and varies according to the system used. The rail pressure is therefore one of the factors that influence the rapidity with which the internal combustion engine can be started.
The pressure in the high-pressure area of the common rail system typically falls rapidly after stoppage and while the engine is being shut down. Here the drop in pressure is mainly caused by leakage points on the injectors, the pump or even any valves fitted.
Various assembly configurations are feasible for fuel injections systems. For example, possible variations include the number of fuel injectors, the number of common rails, additional divided rails, and additional valves, for example, for pressure control.
To restart the engine, the pressure in the rail system must be rapidly raised again to a level at which starting is possible. In currently known systems, the pressure can be increased only by the high-pressure pump already used, which typically is driven via the engine. For various reasons such as cost-saving and energy efficiency, for example, the smallest possible pumps are used. This relates firstly to the overall physical size but also to the volumetric flow that is generated per revolution.
The build-up of pressure by the high-pressure pump crucially depends on the volumetric efficiency of the piston chamber of the high-pressure pump, the air content of the liquid (diesel) and the starting pressure for engine and pump starting.